CirrusCI: Fix the last few FreeBSD failures

[kinke]

As the std.format unittest failure (missing - sign when printing a negative double as hex string) smells like a CTFE issue and might be related to a buggy host druntime's core.stdc.math.log2l.

The previous host compiler was FreeBSD's LDC v1.14, possibly including unknown patches.

[kinke]

• The CTFE issue for std.format was fixed with core.stdc.[tg]math: Revise for FreeBSD dlang/druntime#3119.
• The dmd-testsuite-release failures were all backtrace-related and fixed by switching to FreeBSD's libexecinfo (FreeBSD: switch to using external (libexecinfo) backtrace function like GDC does #3108). Embedding the library dependency via pragma(lib, "execinfo") in druntime works nicely, working for both shared and static druntime without having to touch CMake/ldc2.conf/hardcoded libraries, as long as using lld 9+. Unfortunately, FreeBSD 12.1 (current stable) comes with clang 8 and lld 8 by default.
• When switching to clang 9 (and apparently implicitly to lld 9), the previous single lit-test failure wrt. invalid relocation for TLS vars and dynamic-compile vanished.
• The std.path issue is being fixed in std.path: Fix tilde expansion for HOME=/ dlang/phobos#7510.