Update ChaCha20 ARM asm with sections.

The ChaCha20 ARM asm is generated from GCC. This change updates the GCC
command line to include -ffunction-sections, which causes GCC to put
each function in its own section so that the linker with --gc-sections
can trim unused functions.

Since the file only has a single function, this is a bit useless, but
it'll now be consistent with the other ARM asm.

Change-Id: If12c675700310ea55af817b5433844eeffc9d029
Reviewed-on: https://boringssl-review.googlesource.com/6006
Reviewed-by: Adam Langley <agl@google.com>
2 files changed