[bazel] Improve libc build on macOS

This isn't enough to get everything in libc working, but I started down
this path and these were the first required fixes. I might get back to
more changes later.

Author: keith

utils/bazel/llvm-project-overlay/libc/BUILD.bazel

@@ -742,12 +742,12 @@ libc_support_library(
     deps = [
         ":__support_common",
         ":__support_cpp_bit",
-        ":__support_sign",
         ":__support_cpp_type_traits",
         ":__support_libc_assert",
         ":__support_macros_attributes",
         ":__support_macros_properties_types",
         ":__support_math_extras",
+        ":__support_sign",
         ":__support_uint128",
     ],
 )
@@ -964,11 +964,17 @@ libc_support_library(
 libc_support_library(
     name = "__support_osutil_syscall",
     hdrs = ["src/__support/OSUtil/syscall.h"],
-    textual_hdrs = [
-        "src/__support/OSUtil/linux/syscall.h",
-        "src/__support/OSUtil/linux/aarch64/syscall.h",
-        "src/__support/OSUtil/linux/x86_64/syscall.h",
-    ],
+    textual_hdrs = select({
+        "@platforms//os:macos": [
+            "src/__support/OSUtil/darwin/syscall.h",
+            "src/__support/OSUtil/darwin/arm/syscall.h",
+        ],
+        "//conditions:default": [
+            "src/__support/OSUtil/linux/syscall.h",
+            "src/__support/OSUtil/linux/aarch64/syscall.h",
+            "src/__support/OSUtil/linux/x86_64/syscall.h",
+        ],
+    }),
     deps = [
         ":__support_common",
         ":__support_cpp_bit",
@@ -978,9 +984,10 @@ libc_support_library(
 libc_support_library(
     name = "__support_osutil_io",
     hdrs = ["src/__support/OSUtil/io.h"],
-    textual_hdrs = [
-        "src/__support/OSUtil/linux/io.h",
-    ],
+    textual_hdrs = select({
+        "@platforms//os:macos": ["src/__support/OSUtil/darwin/io.h"],
+        "//conditions:default": ["src/__support/OSUtil/linux/io.h"],
+    }),
     deps = [
         ":__support_common",
         ":__support_cpp_string_view",
@@ -992,10 +999,15 @@ libc_support_library(
 libc_support_library(
     name = "__support_osutil_quick_exit",
     hdrs = ["src/__support/OSUtil/quick_exit.h"],
-    textual_hdrs = [
-        "src/__support/OSUtil/linux/quick_exit.h",
-        #TODO: add support for GPU quick_exit (isn't just in a header.)
-    ],
+    textual_hdrs = select({
+        "@platforms//os:macos": [
+            "src/__support/OSUtil/darwin/quick_exit.h",
+        ],
+        "//conditions:default": [
+            "src/__support/OSUtil/linux/quick_exit.h",
+            #TODO: add support for GPU quick_exit (isn't just in a header.)
+        ],
+    }),
     deps = [
         ":__support_osutil_syscall",
     ],
@@ -1927,7 +1939,12 @@ libc_math_function(name = "copysignf")
 
 libc_math_function(name = "copysignl")
 
-libc_math_function(name = "copysignf128")
+libc_math_function(
+    name = "copysignf128",
+    additional_deps = [
+        ":llvm_libc_types_float128",
+    ],
+)
 
 libc_math_function(name = "ilogb")