[legacy] Allocate legacy driver .bss-like segments at probe time

Some legacy drivers use large static allocations for transmit and
receive buffers.  To avoid bloating the .bss segment, we currently
implement these as a single common symbol named "_shared_bss" (which
is permissible since only one legacy driver may be active at any one
time).

Switch to dynamic allocation of these .bss-like segments, to avoid the
requirement for using common symbols.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/compiler.h

@@ -623,28 +623,6 @@ char __debug_disable(OBJECT) = ( DBGLVL_MAX & ~DBGLVL_DFLT );
  */
 #define inline inline __attribute__ (( no_instrument_function ))
 
-/**
- * Shared data.
- *
- * To save space in the binary when multiple-driver images are
- * compiled, uninitialised data areas can be shared between drivers.
- * This will typically be used to share statically-allocated receive
- * and transmit buffers between drivers.
- *
- * Use as e.g.
- *
- * @code
- *
- *   struct {
- *	char	rx_buf[NUM_RX_BUF][RX_BUF_SIZE];
- *	char	tx_buf[TX_BUF_SIZE];
- *   } my_static_data __shared;
- *
- * @endcode
- *
- */
-#define __shared __asm__ ( "_shared_bss" ) __aligned
-
 #endif /* ASSEMBLY */
 /** @} */