Stuffing all the files together and compiling the resulting blob with
the main program is a terrible idea.

Some files, namely the .S ones, must be run trough the C preprocessor
before assembling them (#2437).

Beside that the aggregate may be mis-compiled due to the presence of
some flags that affect the following code.

For example let's consider two files, a.s and b.s

a.s
```
fn1:
    ret
.data
data1:
    .word 0
```

b.s
```
fn2:
    ret
```

Now, fn1 and fn2 will be both placed in the .text section as intended if
the two files are compiled separately. But if we merge them the `.data`
flag ends up placing fn2 in the wrong section!

This fixes a nasty crash where musl's memset ended up in the
non-executable data segment, leading to too many hours of
head-scratching.