[flang] Fix Funderscoring TC failure on AIX [NFC] (#73543)

Change fundersoring test with a more accurate regex to ensure the lack
of a trailing underscore, current TC is failing on AIX

Co-authored-by: Mark Danial <[email protected]>

Author: madanial0

flang/test/Driver/underscoring.f90

@@ -21,4 +21,4 @@ subroutine test()
 ! NO-UNDERSCORING-NOT: ext_sub_
 ! NO-UNDERSCORING: {{ext_sub[^_]*$}}
 ! NO-UNDERSCORING-NOT: comblk_
-! NO-UNDERSCORING: comblk,
+! NO-UNDERSCORING: {{comblk[^_]*$}}