Base relative variable addressing

Introduction

For m68k the SAS/C compiler had the possibility to use the A4 register as a base address for accessing global variables. This way so called pure programs can be generated, these are programs that can be loaded once into memory and run several times. Every time when then the program is started then room is allocated for the variables and the A4 variable is set to the allocated memory.
Such a feature is also needed on the i386 ABI.

Implementation

Different options have been examined in the past to see how this feature could best be provided on i386. Currently it has been settled on using the %ebx register for this purpose. This is analog to the SYSV i386 ABI where %ebx can be reserved for use as global offset table (GOT) for position independent code.
The register has been reserved for this purpose by adding the -ffixed-ebx option to the gcc specs file. This means that gcc will not generate code that accesses the %ebx register so it is reserved for other system use.
The consequence is that you can't use the "b" constraint anymore on inline asm in gcc for input or output arguments. This constraint would allow putting arguments in the %ebx register but gcc gives an error because it is supposed to not use this register; inline asm statements of external code may thus need to be changed to work on AROS.
Two macros (AROS_GET_RELBASE and AROS_SET_RELBASE) are defined to access this variable. These hase to be defined by cpu.h for a certain cpu otherwise you will get a compile error.
At the moment no patch for gcc is available for using the --baserel option on i386 for producing pure code for AROS. Feel free to implement it.
At the moment on i386 this register is also used for passing the libbase pointer.

Other options

OtherOther things have been considered and some even tested for a good place to store the relbase. Here are some summaries:

__relbase system global variable. Analog currently to the SysBase variable there would be a __relbase global variable where the value is provided by the ELF loader. This approach works but this path was left when %ebx was discovered and it's analogy to the SYSV usage of this register.
%fs:n or %gs:n: Another possiblity would be to use the i386 segment registers for accessing the relbase. Also here %ebx is currently considered a better solution. The impact of using the segment registers on a hosted version of AROS is not fully clear yet.
ThisTask->relbase. Could be an alternative but %ebx seems better.
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