avr-gdb: upgrade to GDB 15.2

[reillylm]

This makes our formula match these other cross-built GDBs in homebrew-core:

• riscv64-elf-gdb
• arm-none-eabi-gdb
• x86_64-elf-gdb
• i386-elf-gdb

Changes from our previous formula:

• GDB now requires GMP (as of 11.1, commit) and MPFR (as of 14, commit)

• remove --prefix=#{prefix} + --disable-debug + --disable-dependency-tracking. These have been added to a reusable std_configure_args (as documented here in the Formula Cookbook).

• remove --disable-install-libbfd. In the bfd configure script, this looks to be disabled if host != target (which is true for us).

• remove --disable-install-libiberty. In the libiberty configure script, this looks to already be disabled by default.

I think we're also avoiding installing libiberty and libbfd by using make install-gdb instead of make install.

• remove --disable-nls + --disable-libssp. I think these flags got copied over from our GCC formulae, but they don't actaully apply to GDB (docs for GCC configure options versus GDB configure options). Same goes for our avr-binutils formula dependency.

Addresses #343, since the newer GDB version has addressed the Clang errors outlined in the issue.

Side note about installation directories

I wanted to know if we actually needed to customize all those installation directories in the configure script flags, so I installed to a custom DESTDIR and checked out the files. Here's what I found (the GNU Makefile Conventions docs were helpful here):

GDB installs executables into bindir, headers into includedir (e.g. for GDB's JIT interface), data files into datadir (Python helper functions, the syscall name database), manpages in mandir, Info files in infodir, and locale files in localedir.

Some installed files have a disambiguating prefix of the target architecture (e.g. avr-gdb), but most files don't, so they would overwrite files from an existing installation of GDB. Therefore, we specify a disambiguating subdirectory for all the installation directories, except: bindir (the executables already have the appropriate prefix) and libdir (install-gdb doesn't install anything here).

mandir needs to be modified, not because the installed manpage files don't have the appropriate prefix (they do), or because they get installed to #{prefix}/man instead of #{prefix}/share/man (like the Formula Cookbook cautions against), but because we're customizing datarootdir, which causes make to install the files to #{prefix}/share/#{target}/man instead of #{prefix}/share/man like they should go. So we need to "undo" the change we made to datarootdir here (as an alternative, I think we could have left datarootdir and mandir alone, and instead customized datadir / includedir / localedir / infodir).