powerpc: Export __parse_hwcap_and_convert_at_platform to libc.a.

Commit 67385a01 added a new feature for
powerpc, where we store HWCAP/Platform bits in the TCB.  In the dynamic
linking case, we use the versioned symbol
'__parse_hwcap_and_convert_at_platform' to verify if this feature is
available.  However, the same symbol was not exported to libc.a, making
it not possible for GCC to check for it prior to link time.