Semihosting SYS_READ call on stdin just returns immediately indicating no data was read, thus rendering stdin unusable with QEMU.

SYS_READC blocks and waits for one symbol to be read from the terminal, however Enter key specifically is returned as '\r', thus the special case (that may not generalize well beyond QEMU).

LLVM libc does not provide a way to redefine abort(), thus the local abort() definition in LLVM libc semihosting has to be removed.

Calls to abort() in LLVM libc crt0 replaced with __llvm_libc_exit() which is the LLVM libc defined platform specific exit handler.

This prevents exception handlers from crashing because of the trap instruction in LLVM libc implementation of abort().

…de (arm#678)

LLVM libc does not provide a way to redefine abort(), thus the local
abort() definition in LLVM libc semihosting has to be removed.

Calls to abort() in LLVM libc crt0 replaced with __llvm_libc_exit()
which is the LLVM libc defined platform specific exit handler.

This prevents exception handlers from crashing because of the trap
instruction in LLVM libc implementation of abort().

Provide init and exit handlers for not hosted environments in llvm libc crt0.

Update the documentation accordingly.

Update the UART sample to make use of the symbols provided in crt0 and refine the comments for the remaining symbols related to IO retargeting.

…embly instructions

Replace direct call to semihosting WRITEC for debug output of exception details to a platform specific handler: _platform_debug_putc(int c)

Implement the handler in semihosting and provide a placeholder in no-host crt0.

…#706)

Add handlers in LLVM libc startup code that can be overridden by the
user to customize hardware and runtime setup, e.g. provide a custom
exceptions table or such.

After the customization handlers were added in
arm@bc8a58b
the optional no-fpregs attribute collided with the extern "C"
declaration on the handler - this was fixed by creating an extern "C"
block around it.

Add support in LLVM libc startup code to get command line options
(argc/argv) from semihosting.

Provide an empty handler that can be overridden for no-host environment.

Semihosting passes the command line options as one string, thus basic
logic to split it into individual arguments implemented:
- Arguments are split by whitespace.
- Quoted text is copied as-is: "a b c " or 'a b c ' will keep all
spaces. Not closed quote will run till the end of the provided line.
- Escape sequences: \ escapes (copies) the next symbol, unless
  inside single quotes or at the end of the string.