Merge bitcoin/bitcoin#23948: build: add support for FreeBSD to depends

ae9175f608 build: add FreeBSD support to depends (fanquake)

Pull request description:

  Setup to use the system Clang (11.0.1 as of FreeBSD 13.0).

  Doesn't build the Qt package; that requires a few additional changes. The current issue is that FreeBSDs `byacc` seems to have an issue parsing something in libxkbcommon. Work in progress branch here: https://github.com/fanquake/bitcoin/tree/depends_support_freebsd_qt. In any case, I don't think building the Qt libs on FreeBSD is a super high priority (I'd also have no way to test the GUI).

ACKs for top commit:
  laanwj:
    ACK ae9175f608

Tree-SHA512: 1ecc7855f0195f68c96e534bd77ce17c3975db1dfb3caa35302c2b46643c8c30f75b1c2e1735cf69f1eddb70447d11e67e7f339ef5497336cdff7a59b32be961

depends/hosts/freebsd.mk

@@ -0,0 +1,31 @@
+freebsd_CFLAGS=-pipe
+freebsd_CFLAGS_CXXFLAGS=$(freebsd_CFLAGS)
+
+freebsd_CFLAGS_release_CFLAGS=-O2
+freebsd_CFLAGS_release_CXXFLAGS=$(freebsd_release_CFLAGS)
+
+freebsd_CFLAGS_debug_CFLAGS=-O1
+freebsd_CFLAGS_debug_CXXFLAGS=$(freebsd_debug_CFLAGS)
+
+ifeq (86,$(findstring 86,$(build_arch)))
+i686_freebsd_CC=clang -m32
+i686_freebsd_CXX=clang++ -m32
+i686_freebsd_AR=ar
+i686_freebsd_RANLIB=ranlib
+i686_freebsd_NM=nm
+i686_freebsd_STRIP=strip
+
+x86_64_freebsd_CC=clang -m64
+x86_64_freebsd_CXX=clang++ -m64
+x86_64_freebsd_AR=ar
+x86_64_freebsd_RANLIB=ranlib
+x86_64_freebsd_NM=nm
+x86_64_freebsd_STRIP=strip
+else
+i686_freebsd_CC=$(default_host_CC) -m32
+i686_freebsd_CXX=$(default_host_CXX) -m32
+x86_64_freebsd_CC=$(default_host_CC) -m64
+x86_64_freebsd_CXX=$(default_host_CXX) -m64
+endif
+
+freebsd_cmake_system=FreeBSD