Put friend decls in the correct context.

When we find a friend declaration we have to skip transparent contexts for doing
lookups, but we should not skip them when inserting the new decl if the lookup
found nothing.

Fixes PR15841.
Adds 64-bit POSIX support for AVX
- Adds unique enums for ymm registers to the ABI and the POSIX register context.
- Reworks the register context data structures to support a union of FXSAVE and XSAVE
--- Allows the same code base to deal with the FPU independent of the availability of AVX.
- Determine if AVX is supported by attempting to read XSAVE using ptrace.
--- Support an extended register set for avx registers if available.
- Provide a mechanism to assemble/parse register halves into a single ymm buffer for use with RegisterValue.
--- Reworked Read/WriteRegister routines to read/write/parse ymm registers.

Adds tests for ymm register write with read-back, and expressions involving ymm registers.
- Tests vary depending on the availability of an avx register set.

Thanks to Daniel and Matt for their reviews.