Allow opting out of thumb mode for ARM builds.
Allow opting out of thumb mode for ARM builds.

Heap profiler relies on frame pointers to do stack unwinding. ARM builds
are using thumb mode by default, but since frame pointers are broken in thumb
mode for both GCC and LLVM (https://llvm.org/bugs/show_bug.cgi?id=18505), we
need a way to build in arm mode.

This change exposes arm_use_thumb GN variable so that it can be set to false.

BUG=602701

Review-Url: https://codereview.chromium.org/1977533002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#393615}

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