Use git mirror instead of mercurial for gmp (#392)

cheribuild/pycheribuild/projects/cross/gmp.py is are using MercurialRepository
with a repository of repository = MercurialRepository("https://gmplib.org/repo/gmp").
However, the host for this (quite rightly) blocks certain large IP ranges, namely
all of Azure and Github Actions runners, this causes any build that has GMP
as a dependency to fail.

There is a well maintained mirror hosted on github at https://github.com/gmp-mirror/gmp
which maintains the entire hg repositories commit history and contains some additional branches.

pycheribuild/projects/cross/gmp.py

@@ -24,12 +24,13 @@
 #
 
 from .crosscompileproject import CrossCompileAutotoolsProject, DefaultInstallDir
-from ..project import MercurialRepository
+from ..project import GitRepository
 from ...config.compilation_targets import CompilationTargets
 
 
 class BuildGmp(CrossCompileAutotoolsProject):
-    repository = MercurialRepository("https://gmplib.org/repo/gmp")
+    repository = GitRepository("https://github.com/gmp-mirror/gmp")
+    default_directory_basename = "libgmp"
     supported_architectures = (
         CompilationTargets.ALL_CHERIBSD_TARGETS_WITH_HYBRID
         + CompilationTargets.ALL_CHERIBSD_HYBRID_FOR_PURECAP_ROOTFS_TARGETS