www/pomerium-envoy-custom: Fix build on aarch64

gperftools has no way to read the program counter from a signal's
ucontext on FreeBSD/aarch64, so its CPU profiler, which Envoy builds
with --define tcmalloc=gperftools, failed to compile:

getpc-inl.h:393:22: error: no member named 'Get' in ...

On FreeBSD/aarch64 the kernel stores the interrupted PC in
uc_mcontext.mc_gpregs.gp_elr. Add an accessor for it to gperftools
and apply the patch through the gperftools http_archive in envoy's
bazel/repositories.bzl, the same way the msgpack patch is applied.
The hunk offsets of the other entries in that patch are refreshed
while here.

The gperftools change has been verified with getpc_test on an Ampere
eMAG running FreeBSD 15.1 and submitted upstream. The full port has
only been test-built on amd64 so far.

Upstream PR: https://github.com/gperftools/gperftools/pull/1647