[Picolibc] Version 1.8.1

[Keith Packard]

### Picolibc version 1.8.1

 * Fix cmake build system to auto-detect compiler characteristics
   instead of assuming the compiler is a recent version of GCC. This
   allows building using cmake with clang.

 * Fix cmake build system to leave out TLS support when TLS is
   disabled on the cmake command line.

 * Replace inline asm with attributes for __weak_reference macro

 * Add allocation attributes to malloc and stdio functions. This
   allows the compiler to detect allocation related mistakes as well
   as perform some additional optimizations. Bugs found by this change
   were also addressed.

 * Add wchar_t support to tinystdio, eliminating the last missing
   feature compared with the legacy stdio bits from newlib. With this,
   libstdc++ can be built with wide char I/O support, eliminating the
   last missing feature there as well.

 * Eliminate use of command line tools when building with a new enough
   version of meson. Thanks to Michael Platings.

 * Add Microblaze support. Thanks to Alp Sayin.

 * Switch semihosting to use binary mode when opening files. Thanks to
   Hardy Griech.

 * Build and install static library versions of the crt0 startup
   code. These allows developers to reference them as libraries on the
   command line instead of needing special compiler support to locate
   the different variants, which is useful when using clang. Thanks to
   Simon Tatham.

 * Simplify the signal/raise implementation to use a single global
   array of signal handlers and to not use getpid and kill, instead
   raise now directly invokes _exit. This makes using assert and abort
   simpler and doesn't cause a large TLS block to be allocated. Thanks
   to Joe Nelson for discovering the use of a TLS variable here.

-- 
-keith
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