Log Message:
gdb: fix installation of gcore.1 on platforms that support gcore

Previously, gcore.1 was only installed on Linux due to an omission
in gdb/doc/Makefile.in. This caused file-check errors on Linux since
gcore.1 was not mentioned in PLIST.common.

To fix this, apply upstream commit [0] so that gcore.1 is installed
whenever gcore is.

[0] \ 
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=4b136f6f9aba2122868704bc18fd21e1d232392c