gdb: update 'Caveats'  (#83892)
gdb: update 'Caveats'  (#83892)

* link to "PermissionsDarwin" on GDB wiki

The instructions to codesign the GDB executable have moved from
https://sourceware.org/gdb/wiki/BuildingOnDarwin to
https://sourceware.org/gdb/wiki/PermissionsDarwin. The former still
exists but links to the later.

Refer users to the new page in the Caveats.

* drop old 'startup-with-shell' workaround

The Caveats section mentions that 'set startup-with-shell off' is needed
on macOS 10.12 and later with SIP. This was added in 3b74e45d744 (gdb:
fix for Sierra SIP, 2016-11-10) when GDB 7.12 was released.

GDB later learned to cache a copy of the user's shell to work around SIP in
b50a8b9a916 (Cache a copy of the user's shell on macOS, 2018-09-17) [1],
which was released in GDB 8.3.1. As such, it's not necessary to set
'startup-with-shell' in one's ~/.gdbinit. GDB will itself tell the user
to set that config if caching the user's shell fail.

1. https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=b50a8b9a916ea2fe1379bcd8f122feef8129a0e9
makigumo/homebrew-core,Homebrew/homebrew-core,Moisan/homebrew-core,mfikes/homebrew-core,sjackman/homebrew-core,chrisfinazzo/homebrew-core,lemzwerg/homebrew-core,mbcoguno/homebrew-core,mfikes/homebrew-core,filcab/homebrew-core,adam-moss/homebrew-core,makigumo/homebrew-core,mbcoguno/homebrew-core,uyjulian/homebrew-core,zyedidia/homebrew-core,nbari/homebrew-core,filcab/homebrew-core,zyedidia/homebrew-core,sjackman/homebrew-core,adam-moss/homebrew-core,stgraber/homebrew-core,Homebrew/homebrew-core,nandub/homebrew-core,cblecker/homebrew-core,nandub/homebrew-core,nbari/homebrew-core,Moisan/homebrew-core,lemzwerg/homebrew-core,stgraber/homebrew-core,mbcoguno/homebrew-core,uyjulian/homebrew-core,chrisfinazzo/homebrew-core,cblecker/homebrew-core