oldest_cpu: Bottling architecture of ARM is armv6

[sjackman]

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This PR is required for brew portable-bottle ruby on 32-bit ARM.

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armv6 is the oldest supported 32-bit ARM architecture
armv8-a is the oldest 64-bit ARM architecture.

See https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html
and https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html

[MikeMcQuaid]

Thanks again @sjackman!

[sjackman]

No worries at all. Thanks for the quick merge, Mike!