devel/libunwind: Fix build on 14-CURRENT on armv6/armv7

ptrace/_UPT_access_fpreg.c:107:25: error: no member named 'fpr' in 'struct fpreg'
          memcpy(&fpreg.fpr[reg], val, sizeof(unw_fpreg_t));
                  ~~~~~ ^
ptrace/_UPT_access_fpreg.c:123:30: error: no member named 'fpr' in 'struct fpreg'
          memcpy(val, &fpreg.fpr[reg], sizeof(unw_fpreg_t));
                       ~~~~~ ^
2 errors generated.

Reported by:	Mark Millard <marklmi@yahoo.com>
Obtained from:	https://github.com/libunwind/libunwind/commit/99fb31d5f05ff6c96ca3a9350c792618fab23f2d
Reference:	https://pkg-status.freebsd.org/beefy8/data/main-armv6-default/pdf5b273c8e36_s83d5725005/logs/libunwind-20211201_2.log
		https://pkg-status.freebsd.org/ampere2/data/main-armv7-default/p519c178336e5_scb3c414f2f/logs/libunwind-20211201_2.log