package/gcc: fortran support now requires MMU

Since upstream commit
c66d1ba685b0d03b28967d2f7a48f6c64687efa2 ("Fortran: Add a shared
memory multi process coarray implementation [PR88076]"), libgfortran
now unconditionally uses fork(), which makes it impossible to build
Fortran support on noMMU architectures.

In follow-up commit efb497c23a6f4109bf8794342617cbc161c08301, it was
made even more clear: if you don't have fork() and you're not on
WIN32:

+#else
+#error "no way known to start child processes."
+#endif

There are apparently no autobuilder failures for this issue, but the
simple defconfig below allows to trigger the build failure:

BR2_arm=y
BR2_cortex_m7=y
BR2_GCC_VERSION_16_X=y
BR2_TOOLCHAIN_BUILDROOT_FORTRAN=y

Even though this issue is only starting from GCC 16.x, we make Fortran
unavailable on noMMU configurations regardless of the GCC version,
with the reasoning that:

1. Fortran on noMMU is unlikely to be in use

2. Moving forward, all future versions of GCC will have this
   restriction

Cc: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/gcc/Config.in.host

@@ -98,6 +98,8 @@ config BR2_TOOLCHAIN_BUILDROOT_FORTRAN
 	# on architecture building libquadmath, wchar is required
 	depends on !BR2_TOOLCHAIN_HAS_LIBQUADMATH || \
 		(BR2_TOOLCHAIN_HAS_LIBQUADMATH && BR2_USE_WCHAR)
+	# requires fork() since gcc 16.x
+	depends on BR2_USE_MMU
 	select BR2_TOOLCHAIN_HAS_FORTRAN
 	help
 	  Enable this option if you want your toolchain to support the