use gcc assembly for arm 6

[studavekar]

Description

fix the armc6 compile error, ARM 6 is compatible with GCC so unify GCC and ARM 6 code

00:00:44.012         [DEBUG] Output: ./TESTS/mbed_hal/flash/functional_tests/main.cpp:82:11: error: no flag-preserving variant of this instruction available
00:00:44.012         [DEBUG] Output:     "%=:\n\t"
00:00:44.013         [DEBUG] Output:           ^
00:00:44.013         [DEBUG] Output: <inline asm>:2:2: note: instantiated into assembly here
00:00:44.013         [DEBUG] Output:         SUB  r0, #1

__asm behavior between arm 5 and arm 6

arm compiler 6.8 :

1. For embedded assembly, you cannot use the __asm keyword on the function declaration.
2. The __asm keyword can incorporate inline GCC syntax assembly code into a function
   reference 6.8

arm compiler 5.06:

The __asm keyword can declare or define an embedded assembly function reference 5.06

Related PRs

#4949

[studavekar]

/morph test

[0xc0170]

LGTM, but we should use toolchain generic macros

[studavekar]

/morph test

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 1285

All builds and test passed!

[mbed-bot]

Result: SUCCESS

Your command has finished executing! Here's what you wrote!

/morph test

Output

mbed Build Number: 1286

All builds and test passed!

[adbridge]

It looks like this will work but I don't usually like #elif without a corresponding default #else case. I think we can live with it for now though