auxv: convert auxiliary vector into an array; define getauxval()

Convert the ELF auxiliary vector into an array.  Define getauxval() as
an inline accessor to that array.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
[klibc] i386: use the vdso for system calls on i386

Use the system call entry point in the vdso for system calls on i386.
The vdso can provide either int $0x80, sysenter or syscall depending
on the CPU.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>